Cyclic polyphosphonitrilic halides can be reacted with alkali metal alkoxides and aryloxides to form derivatives that can be used as hydraulic fluids, flame retardants, or as plasticizers for high molecular weight polyphosphazenes. The most common cyclic polyphosphonitrilic halide is phosphonitrilic chloride trimer. A major use for trimer is in the manufacture of high molecular weight substantially linear polyphosphonitrilic chloride. The high molecular weight polymers can be obtained by merely heating the purified trimer at about 220.degree.-300.degree. C. for a period of 12-48 hours. Preferably the polymerization is conducted in the presence of a very small amount of a Lewis Acid catalyst, e.g. AlCl.sub.3, BCl.sub.3 and the like. The linear high molecular weight polyphosphonitrilic chloride can be substituted by reaction with alkali metal alkoxides and/or aryloxides to form elastomers having very useful physical properties. These elastomers can be compounded and cured to form useful articles of manufacture. The conventional method for making cyclic phosphonitrilic chloride is to react phosphorus pentachloride with an excess of ammonium chloride in a suitable liquid reaction medium (e.g. monochlorobenzene or sym-tetrachloroethane) at a temperature of about 120.degree.-150.degree. C. This reaction takes several hours to complete and yields a mixture that is about 75-80% trimer. A process has been reported by W. Sulkowski et al., Chem. Stos 26 (2) p. 245-252 (1982), in which pyridine is added to a well-ground mixture of phosphorus pentachloride and ammonium chloride causing the temperature to rise sharply to 180.degree. C. causing strong boiling presumeably of the pyridine. The reaction is complete in minutes giving a low yield of cyclic phosphonitrilic chloride, product consisting mainly of trimer and tetramer. The reaction can also be conducted in sym-tetrachloroethane using quinoline as the nitrogenous base. Although this process is very rapid, it is not practical on a large scale because the large exotherm over such a short reaction period leads to an uncontrollable reaction. Also the yield of cyclics is low.
In U.S. patent application Ser. No. 756,799, filed July 19, 1985 by H. R. Allcock, S. J. Stinnett, J. B. Tedder, Jr. and J. R. Adams, Jr., a process for making cyclic phosphonitrilic chloride trimer is disclosed in which phosphorus pentachloride is first reacted with a nitrogenous base (e.g. pyridine) to form a complex and then ammonium chloride is added and the mixture reacted at a temperature above 90.degree. C. Alternatively both phosphorus pentachloride and ammonium halide can be placed in the nitrogenous base and reacted at a temperature below about 70.degree. C. and the mixture then heated to a temperature above 90.degree. C. to initiate a second reaction which forms cyclic phosphonitrilic chloride.
Although the above process is rapid and gives good yields of trimer, it suffers the economic penalty of providing no means to recover or reuse pyridine. A need exists for a process that will give the same high yield and reaction rate of Allcock et al. and at the same time conserve the nitrogenous base.